PFL and AFL are solo functions (pre-fade and after-fade). With PFL, you hear the original signal regardless of how the fader is set. The AFL function depends on fader setting. So AFL is routed to the fader buss, whereas PFL is routed directly to the output, bypassing the faders (my terminology may be different from the class').

I don't know about that 24/25 channel question. The mixer I used only had 24 channels.